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86 gmc s-15 a/c prob

r-12isverycold on Thu July 23, 2009 3:25 AM User is offline

Year: 1986
Make: GMC
Model: S-15
Engine Size: 2.8 V-6
Refrigerant Type: R-12
Ambient Temp: 95
Pressure Low: 30ish
Pressure High: n/a
Country of Origin: United States

Got a 86gmcS-15 2.8 tbi v6 and the a/c compressor sometimes seems to lockup.
it squeals the belt and wont turn, the low side pressures (when its working properly) is around 30 psi (R12 refrigerant ).
when its working (which is most of the time) the compressors quiet i get nice cold a/c.
I've also changed the a/c cycle switch(brand new meant for r12 from vautozone.
i can get the a/c compressor freed by switching on and off the a/c a few times then it works fine , when it locks up it will spin a few time then screeching belt is heard.


Chick on Thu July 23, 2009 6:57 AM User is offlineView users profile

You really need both high and low pressures, but here's a couple thoughts, bad belt and/or bad tensioner, (most likely if the compressor is good) making it lock up when the high side pressure gets to high, how does it work on the highway with good air flow keeping the high side down?? Clutch fan may also need to be replaced, but get both pressures when it running fine and when it locks up (high side right before it locks up)

Email: Chick


Freedoms just another word for nothing left to lose

GM Tech on Thu July 23, 2009 8:19 AM User is offline

It is probably not locking up-- it is just experiencing high load, high torque transfer due to lots of heat to remove, higher head pressure-- does this only happen when it is the hottest out? Your belt tensioner is in question- probably weak- and the belt is slipping over the compressor pulley. If your compressor was locking up- it would not run again----and cool again.

If your alternator belt was slipping, you would not say the alternator is locking up- same scenario with the compressor..

You may have a belt that is too long-- if you look hard at the tensioner, there is a "window" that shows proper "zone" the tensioner will run in- it has an arrow that should fall between two end points- is your tensioner in this zone? Also, I have seen wrong belts actually wrapped wrong, causing less belt tension than necessary..

I actually own and '86 S-15 BY THE WAY.

I get the same compalints on a v-belt system-- but there we just tighten the v-belt....

Check the fan clutch- and high side pressures when this happens

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

JLHAWKINS on Thu July 23, 2009 2:17 PM User is offline

Like both Chick and GM Tech suggested, check the fan clutch also. I will share my experience with a similar vehicle.

I have a 1985 S-10 Blazer with the 2.8L and the A/C converted to 134A that I have owned for about 20 years. It had a problem with high pressure on hot days, never a slipping belt nor a slow down in compressor speed but just not good overall system performance. Condensor and everything else was a clean as I could get it. Fan clutch (original 250K miles) seemed to work OK as the vehicle never overheated nor ran hot. After reading this forum concerning air flow and the effect of the fan clutch on pressures, I finally broke down and replaced the fan clutch. What a difference! At 1500 rpm and 90 degrees ambient, the high pressure fell from 325 down to 250. Vent temperatures fell similarly. Fan noise from the vehicle now sounds like a small plane taxiing on the runway where before it was almost non-existant. If you can't hear your fan then your fan clutch probably needs replacing.

Poor airflow caused by a worn fan clutch can drive up your presssures to the point that it puts alot of stress on the drive belt and if every is not just so-so, slippage can occur.

Anyway, I am now a true beliver in having a good fan clutch. You might want to check yours.

r-12isverycold on Thu July 23, 2009 11:40 PM User is offline

Thanks for replying to my post.
my s-15 still has the VEE belts not serpentine ones(i wish it did though)
a total of three belts, and only the a/c one screams, but only when the engaged clutch on the a/c comp .
its very odd that it does this, it cools quite well. if i flip the a/c swith on and of a few time it unlocks the comp. ive visually seen the comp stop moving and screech the belt.
it did it to me in traffic the other day. and it only seems to do it after the a/c's been run for a long time. even after sitting over night

other then some of the air flow doors not moving, tons of cold air blow out the heater vent, ive ductaped it closed for now. i get fairly good air flow from the dash vents now, all except the far right passenger vent.

my fan clutch moves lots of air and is seems to be working properly.

i've only got one of those pencil gauges so i don't know if i can ge the high side pressures for you
I just bought this truck a few weeks ago, it gets much better mpg than my big ole fullsize]
( 1/2 ton gmc 5.7tbi)
the s-15 had been sitting for 6mths or so, needed a new ignition module and switch.
see pics

thanks again

Dougflas on Fri July 24, 2009 5:24 AM User is offline

Beg, borrow, or rent a set of real gages. Try autozone for rental. You are wasting your time without proper gages and pressures. You may even be wasting money by guessing the cause of the problem.

bohica2xo on Fri July 24, 2009 9:54 AM User is offline

We have had serpentine belts for so long now, sometimes we forget all of the "old" problems.

This may be as simple as replacing the belt. Back in the days of Vee belts, people would tension the crap out of one that was slipping. This of course kills the bearings in water pumps, alternators, etc. - but may not fix the problem. An old, hard belt will glaze up on the surface, and just quit driving - and start squealing. It will come & go at first. Put a new belt on it, and make sure nothing is leaking (oil, coolant etc.) onto the belt path.


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